A matrix method was developed to examine functional interactions between brain regions, by correlating the cerebral metabolic rates for glucose as determined by positron emission tomography in humans. The method was applied to regional metabolic data from 40 healthy men at rest, and demonstrated correlations among homologous regions between the cerebral hemispheres, and between the frontal and the parietal lobes on the one hand and the temporal and occipital lobes on the other. Furthermore, right-hemispheric regional interactions exceeded those in the left hemisphere. The method also showed clear evidence of age differences between 15 young and 15 older men. The older subjects had fewer significant correlations in parietal and frontal regions, indicative of reduced integrated activity in these regions with age. The matrix method was applied to analyze glucose metabolism in Fischer-344 rats, and showed patterns of correlations similar to those in humans.